add strings

 1from string import digits
 2
 3class Solution:
 4    def addStrings(self, num1: str, num2: str) -> str:
 5      output = ''
 6      ai, bi, carry = 0, 0, 0
 7      while ai < len(num1) or bi < len(num2) or carry:
 8        n1v = digits.index(num1[~ai]) if ai < len(num1) else 0
 9        n2v = digits.index(num2[~bi]) if bi < len(num2) else 0
10        carry, v = divmod(n1v + n2v + carry, 10)
11        output = digits[v] + output
12        ai, bi = ai+1, bi+1
13      return output
14
15assert Solution().addStrings('123', '458') == '581'